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The SAD’s and MOV’s speed is due partially to
their high capacitance. Surge current charges the
capacitance, making the effective response time of
a leadless MOV less than a nanosecond. A large
pulse-handling SAD may have high capacitance. A
leadless chip SAD reacts in picoseconds.
SADs and MOVs are rarely used without leads,
even though leads add inductance. Leads must
be connected to these devices when they are
used in a protector. Protectors using either SAD or
MOV components are often advertised as having
sub-nanosecond response times. To accomplish
this, they must use lowpass filtering to offset the
inductive lag, insuring a quick response. Power line
protectors advertised as having sub-nanosecond
response time have misleading specifications if
they do not have lowpass filtering or leadless design
and installation. Filtering is important to prevent small spikes, surges and noise. Although the voltage
excursion of such spikes, surges and noise may not be hazardous, it may cause equipment problems.
Another device listed is a four-layer semiconductor. It is a “follower” or “negative resistance” device,
much like the SCR. It handles more current than a SAD of the same size because of its crowbar action
and unlike the SCR, it has a “turn-off” voltage.
Four-layer semiconductor protection devices are not limited to power line applications. They may also be
used on telephone or control lines, alone or in complex combinations (hybrids).
The last device on the list which could be applied in a protection circuit is the SCR (Silicon Controlled
Rectifier). This device comes in a variety of sizes and could be very fast when teamed up with an SAD or
MOV. The SAD/MOV provides the speed and the SCR protects the SAD/MOV from long duration surges.
In power main applications, the SCR’s dv/dt problems could be buffered by the capacitance of the SAD/
MOV and any filtering that is present.
AC Main Power Protector Types
Parallel or Shunt Type Protector. The shunt type ac protector is the most common protector circuit in
use today. It can be a simple MOV from each lead to the safety ground, matched MOVs or SADs in a
differential and/or common mode, or a hybrid circuit of cascading components for fast turn-on and high
surge current capability. The shunt type protector is not load dependent and must be located close to the
entrance panel to reduce propagation delays. A 10kA rated protector in a typical installation will rarely
conduct anything close to its current rating except during a direct strike to the secondary drop at the
entrance panel. The amount of current the protector will conduct to ground depends on the equipment
load impedance, the inductance of the ground conductor on its way to earth ground, and the fall of
potential resistance/impedance of the earth ground system.
Series In-Line Type Protector. An In-Line ac power protector consists of a pair of protective devices
or hybrid circuits per phase, connected line to ground, one on each end of a load bearing air wound (air
will not saturate) series inductor. There should be a circuit breaker or fuse in series with each non-linear
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